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We have identified a Mn-rich layer on the surface on (Ga,Mn)As thin films which significantly influences soft x-ray absorption measurements. The Mn L3,2 x-ray absorption spectra of the untreated films show a strong multiplet structure, consistent with earlier observations and characteristic of MnO. After removal of the surface layer, the multiplet structure is less pronounced and the spectrum is shifted to ∼0.5 eV lower photon energy. Comparison with calculated spectra imply a localized Mn ground state for the untreated sample and a hybridized ground state after etching. In addition, a large x-ray magnetic circular dichroism is observed at the Mn L3,2 edge in the etched film. These results may explain several peculiarities of previously reported x-ray absorption studies from (Ga,Mn)As.

We report on a comprehensive study of the ferromagnetic moment per Mn atom in ͑Ga,Mn͒As ferromagnetic semiconductors. Theoretical discussion is based on microscopic calculations and on an effective model of Mn local moments antiferromagnetically coupled to valence band hole spins. The validity of the effective model over the range of doping studied is assessed by comparing with microscopic tight-binding/coherent-potential approximation calculations. Using the virtual crystal k · p model for hole states, we evaluate the zerotemperature mean-field contributions to the magnetization from the hole kinetic and exchange energies, and magnetization suppression due to quantum fluctuations of Mn moment orientations around their mean-field ground state values. Experimental low-temperature ferromagnetic moments per Mn are obtained by superconducting quantum interference device and x-ray magnetic circular dichroism measurements in a series of ͑Ga,Mn͒As semiconductors with nominal Mn doping ranging from ϳ2 to 8%. Hall measurements in as-grown and annealed samples are used to estimate the number of uncompensated substitutional Mn moments. Based on our comparison between experiment and theory we conclude that all these Mn moments in high quality ͑Ga,Mn͒As materials have nearly parallel ground state alignment.

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Using x-ray magnetic circular dichroism ͑XMCD͒ the vanadium and cobalt magnetic moments at the V/Co interface have been extracted for different V and Co thicknesses. We find a large magnetic moment induced on the V that is coupled antiferromagnetic to the Co. For Co both the spin and orbital moments are reduced at the interface. The evolution of the magnetic moment together with the spectral changes in the V L 2,3 XMCD upon V deposition demonstrate the complexity of the V/Co interface as manifested in its physical properties. We show how the hybridization at the interface between the Co and V can lead to a transfer of the exchange interaction, resulting in the modified magnetic moments. The change in the number of Co 3d holes, extracted from x-ray absorption spectroscopy, shows that the charge transfer over the interface is small, indicating that it can not be responsible for the change in the magnetic moments.

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The element-specific technique of x-ray magnetic circular dichroism (XMCD) is used to directly determine the magnitude and character of the valence band orbital magnetic moments in (III,Mn)As ferromagnetic semiconductors. A distinct dichroism is observed at the As K absorption edge, yielding an As 4p orbital magnetic moment of around -0.1 Bohr magnetons per valence band hole. This is strongly influenced by strain, indicating its crucial influence on the magnetic anisotropy. The dichroism at the Ga K edge is much weaker. The K edge XMCD signals for Mn and As both have positive sign, which indicates the important contribution of Mn 4p states to the Mn K edge spectra.Comment: 5 pages, 2 figure